A variety of different printing methods for use in printing machines are known. Such printing methods include, for example, printing methods that do not involve a fixed printing forme. This enables each printed product to be produced individually. As a result, personalized printed products can be produced and/or, since printing formes are dispensed with, small print runs of printed products can be produced at low cost. One such printing method is inkjet printing or ink-jet printing. In this method, individual droplets of coating medium are ejected through nozzles of print heads and are transferred to a printing material so as to produce a printed image on the printing material. By actuating a plurality of nozzles individually, different printed images can be produced.
The precise alignment of printed images on the front and back sides of a printing material that is printed on both sides is referred to as register (DIN 16500-2). In multicolor printing, the merging and precise correlation of individual printed images of different colors to form a single image is referred to as color-to-color registration (DIN 16500-2). Suitable measures are also necessary in inkjet printing in order to maintain color-to-color registration and/or register.
EP 2 202 081 A1 and JP 2003-063707 A each disclose a printing machine, wherein the printing machine comprises a first printing unit and a dryer, and the first printing unit comprises a central cylinder with a separate drive motor assigned to the first central cylinder and at least one inkjet print head.
DE 10 2011 076 899 A1 discloses a printing machine which has at least one printing unit and at least one print head embodied as an inkjet print head.
In inkjet printing, print heads which customarily each have a plurality of nozzles are used. Depending on the printing image, for example depending on the division thereof into color separations, various nozzles will not eject any coating medium for a longer or shorter period of time. Sometimes, when a nozzle has been in resting mode for an extended period of time, the nozzle will not react properly the next time it is activated and, for example, will eject at least one required droplet too late or not at all. This may be caused, for example, by changes in the coating medium when it comes in contact with the ambient air in the region of the nozzle and/or by changes in the nozzle itself, for example by changes in temperatures or by the decay of vibrations of individual components. Methods are known in which, in addition to desired printing images, predetermined preventive maintenance printing images are produced at regular intervals to perform preventive maintenance on all nozzles, and thus to achieve a constant readiness to print on demand at any time. Such preventive maintenance printing images may be strips, for example, which are printed between individual complete printing images and are optionally cut off of the printing material during post-processing. From EP 2 390 101 A2, a method is known in which, to maintain print quality in an inkjet printing process, additional droplets are ejected in addition to the printing image. It is also known to superimpose the actual printing image with an additional printing image, which appears as a finely dispersed pattern of dots in the background of the actual printing image, and for which each nozzle is actuated at least once.
A printed product in the above and in the following is understood particularly as a finished product which is printed and is optionally folded and/or cut to size. Various types of printed products exist, for example printed products consisting of only a single page. In that case, for example, sheets of printing material of the proper size are imprinted, or webs of printing material are imprinted and then cut. Other types of printed products comprise a plurality of individual pages that are printed on at least one common printing material, in particular at least one common printing material web, in which case the printing material is then folded, for example in at least one folding device, and cut. Depending on the type of folding device and/or the method for controlling said at least one device, the individual pages must be printed in a certain arrangement on the printing material so that, once they have been folded, a correct orientation and sequence of the individual pages is ensured. This specific arrangement of individual pages is called a complete printing image or signature, for example. A complete printing image can consist of two rows of four separate pages each, for example, wherein the individual pages of a first of the two rows are oriented upside-down, for example, as compared with the individual pages of a second of the two rows. A first complete printing image is applied to the printing material by means of a first printing unit, for example, and a second complete printing image is correspondingly applied to the printing material by means of a second printing unit, particularly correspondingly to a back side of the printing material. Additionally or alternatively, a complete printing image has a continuous printing image or motif which extends over substantially the entire printed product. This may also be the case with strip-shaped printing material. For example, a complete printing image may extend over substantially the entire width of the printing material.
Frequently, as the printing material is being transported between the first printing unit and the second printing unit, a deformation of the printing material can occur, for example a shrinkage due to a loss of moisture resulting from an intermediate drying process and/or a stretching resulting from a softening of the printing material by solvent and/or water that is applied along with the coating medium. A relative change in the corresponding dimensions of the printing material can be as much as 1% (one percent), for example. This can result in a registration and/or register that no longer meet the standards for quality. Such effects may have a more or less serious impact in different directions, for example, depending on the alignment of the paper fibers of the printing material, in particular they may have a less serious impact in the transport direction of the printing material than in the direction transversely to this transport direction.
From WO 2009/005766 A2 a printing machine is known which has two printing units with print heads aligned toward the same side of the printing material.
From WO 2005/031470 A1 a method is known in which toner images are produced based on bitmaps and in which, based on a projected shrinkage of a printing material, these bitmaps are modified during their generation with respect to the number of pixels to be printed.
From DE 101 11 216 A1 a method is known in which image data are modified to compensate for heat shrinkage, wherein an amount of heat shrinkage is first obtained through experimentation and corresponding data are stored in a memory.
From DE 10 2007 040 402 A1 a method is known in which changes in the dimensions of a printing material are compensated for by placing inkjet print heads in an inclined position.
From DE 10 2009 051 197 A1 a method is known in which rastered image data for inkjet printing are modified to compensate for web shrinkage.
From U.S. Pat. No. 4,721,969 a method is known in which image data for a thermal transfer printer are displaced or stretched to compensate for changes in the dimensions of a printing material.
From U.S. Pat. No. 2,010,171 975 A1 a method is known in which sheet-type printed products are produced by dividing a printed image up among a plurality of sheets since it is larger than the sheets at hand.
From U.S. Pat. No. 2,011,304 886 A1 a method is known in which the alignment of printed images on a front side and a back side of sheets is synchronized.
U.S. Pat. No. 2,010,321 429 A1 discloses an inkjet printing method in which ink droplets of different sizes are used.